I was a pseudo vegetarian for 10 yrs between 1995 and 2005. I ate fish.
Since then I discovered that man's rational mind historically developed from the subhuman level because of his diet change from nuts and berries to meat, the former not supplying enough nutrition to feed a rational brain.
However today, protein isolation is widely available and one does not need to eat tons of agricultural produce to extract the necessary amounts of protein to survive well.
So yes, it is possible, but ironically, only because of technology, the product of a rational mind.

Why does one choose to be a vegetarian? If you choose it because you want to live ascetically, many things are wrong, including your diet. If you want to sacrifice your enjoyment to animals you don't know (and which would never have existed if men wouldn't eat meat) - that is wrong. If you choose it because you simply don't like meat - well, the answer is obvious. Even Kant wouldn't tell you that you have a duty to eat a steak once in a while;)

I chose it at the time because I believed it my healthiest choice.
I was a body builder and a cyclist and so my physique, power and endurance were very important to me. One day my wife and I decided to go on a 'purge'. We dieted on water alone for 7 days. We read a story of one such couple that celebrated successfully getting through the week with going out and having a steak dinner. The man died of a heart attack right there.
Because of this we decided to ease back into normalcy. The 1st day we had only fruits, juices and bland crackers. The 2nd, we chose white meat..

..and fish. I had had fish on 2 occasions that day and it digested fully and very quickly with no problem. At 6pm I had 2 very thin slices of white meat and the whole room started to spin in about 2 mins.
I realized that my system was very sensitive and that it was probably telling me the truth. I knew that if I continued, I would most likely get used to it but I wanted to listen to my body and feed it perfectly.
So I decided right there that I would become a fish eating vegetarian - FOR HEALTH REASONS.
In my case therefore, it was a moral decision. Because of this, I was not even tempted.

It lasted for 10 yrs until I found out what I wrote about. Since I believed what I found out to be true, it was no longer a moral decision to stay on the diet. I do still however stay away from much red meat. Again for health purposes.

There are two broad types of argument advanced in support of vegetarianism: moral and nutritional. The moral argument advocates vegetarianism on the grounds that eating meat necessitates treating animals in a way that violates moral principles -- animal rights, cruelty, etc. The nutritional argument advocates vegetarianism on the grounds that it better serves human dietary needs.

Objectivism rejects the moral arguments in support of vegetarianism, but takes no position one way or the other on the nutritional ones. What kind of diet best meets human health requirements is an empirical, scientific question, not a philosophical one.

There are a number of prominent Objectivists who advocate the "paleo" diet, which is definitely not vegetarian, but that advocacy is distinct from their philosophy.

For those interested in the nutritional arguments favoring or opposing a vegetarian diet, I have offered my own thoughts in past discussions, notably here. In that posting, I provide weblinks to my own principal source of information about a vegetarian diet (along with a disclaimer about certain unfortunate political views also endorsed by that doctor). For reference, here are those weblinks again: main link, and newsletter archive.

Anyone who is not actually suffering from any health issues attributable to a meat-based diet probably will have little incentive to give up meat eating. With advancing age, however, one may find that a vegetarian diet, properly done, actually does offer real nutritional benefits over a meat-based diet. It may be difficult at first to understand how a vegetarian diet could meet human nutritional needs. From my own study and actual implementation in my own experience (for nearly 10 years now), I have confirmed that following a diet that mixes minimally refined, naturally starchy foods (like potatoes, brown rice, wheat bread for those who are not sensitive to gluten, beans, etc.) with plenty of vegetables and moderate fruit can be extremely effective in relieving digestive problems, achieving and maintaining optimal body weight, providing adequate energy calories for healthy functioning without overloading one's digestive system with too many calories, and providing nearly all of the essential micronutrients that the human body needs. (One micronutrient supplement that long-time vegetarians are typically advised to take, as a precaution, is vitamin B12.)

The biggest single problem with the typical Western diet is the ratio of calories to total food weight. The key to solving the problem is to eat foods that are much lower in calories for comparable food weight. Then one can eat more of those foods, yet still loose weight (or not gain it), because one's effective calorie intake is so greatly reduced. One can consume surprisingly large portion sizes without the huge calorie intake that comes with it in the typical Western diet.

So many misconceptions about human nutritional needs have been thoroughly refuted in the vegetarian nutritional literature, such as the alleged need for protein. Man actually needs amino acids, which can be obtained from plants (as plant-eating animals do). Man does not need any animal protein. When animal protein is consumed by humans, the human body simply breaks the animal proteins down into amino acids, then synthesizes the human proteins that a human body needs. All one needs to do for human health is obtain all the essential amino acids, either from plants or from animal proteins.

For energy calories, the major sources are fats, carbohydrates, and proteins. Minimally refined starches provide calories from carbohydrates in an easily digestable form (along with abundant indigestable carbohydrates that increase total food weight), while not overloading the human body with too many calories in relation to total food weight. The human body also has a naturial metabolic "bias" toward burning carbohydrates first and storing fats for later whenever both are consumed in the diet. A plant-based diet that mixes minimally refined carbohydrates (starches) with vegetables and fruits can provide an optimal quantity of carbohydrate calories along with relatively low additional fat calories, thereby giving man adequate energy without excessive calorie intake.

Note that this type of vegetarian diet does not mean trying to meet all of one's daily nutritional needs from vegetables and fruits alone. One cannot obtain adequate energy calories that way, and the mix will be vastly harder to digest without the accompanying carbohydrates from minimally refined starches.

Morally, the only objectively relevant issue is what works best for one's own life (and whether or not one is evading relevant evidence). For those suffering from health issues, there is much to be said nutritionally for a sensible vegetarian approach (a starch-based diet, with the addition of vegetables and fruit).

Good answer about the food caloric to food weight ratio.
Just be aware that weight is not a fundamental indicator of good health. Fat ratio is.
Americans throw around the 'weight' word loosely. The BMI index is not an indicator. Competitive bodybuilders fail a BMI for health insurance.
When I get in shape, I GAIN weight and I want to. I gain muscle mass and lose fat. Since muscle is more dense (weighs more for a given volume) than fat, the result is not surprising.
Read this.

Thanks, Ideas for Life. You gave me new facts to consider. I don't think my particular health situation needs these actions but I respect the effort you've put into the research. Thanks. (I'll delete this comment after you've had a chance to read it.)

I want to pose the question: Is it rational to be a vegetarian? I don't think so. So my answer to the question is your form is YES, there is something wrong with following a vegetarian diet. Because it contradicts the nature of us as human beings (as omnivores) that have evolved over millions of years that require meat and protein for healthy survival. Because animals in other species do not have the same rights to exist as people. Because it is possible to be ethical in every other aspect and still eat meat.
If there are valid reasons and arguments to say it is rational to be a vegetarian I would like to hear them. I think many well-intentioned people are motivated to be vegetarians because of opposition to modern meat-production techniques. But that is a separate issue and shouldn't prevent someone from acting in their rational self-interest, which I believe means acting consistent with their nature. And as a human being, that means being an omnivore with respect to one's diet.

Let me try again--I know of two primary reasons to be a vegetarian and I don't think either one is rational. The other is the belief that it's healthier. I believe that contradicts and evades the fact that humans have evolved over millions of years to be omnivores. (The first reason being that it's wrong to kill animals to eat them, which is also mistaken.)